There is an increasing demand for battery systems which are intended to be used in stationary applications such as wind power installations and standby power supply systems, or else in vehicles. All these applications place stringent requirements on the reliability and fail-safety. The reason for this is that complete failure of the voltage supply through the battery system can lead to a failure of the overall system. For example, in wind power installations, batteries are used in order to adjust the rotor blades when the wind is strong and to protect the installation against excessive mechanical loads, which could damage or even destroy the wind power installation. If the battery in an electric vehicle were to fail, it would become undriveable. A standby power supply system is in turn actually intended to ensure interruption-free operation, for example, of a hospital and therefore, as far as possible, cannot fail itself.
In order to make it possible to provide the power and energy required for the respective application, individual battery cells are connected in series, and in some cases additionally in parallel. FIG. 1 shows an outline circuit diagram of batteries connected in series. A multiplicity of battery cells 10-1 to 10-n are connected in series in order to achieve the high operating voltage, as required for example for the electric motor in a passenger car, by addition of the voltages of the individual cells 10-1, . . . , 10-n. The high operating voltage can be decoupled by output-side switches 11-1 and 11-2 from the downstream power-electronic components, such as inverters, which are not illustrated. Since the total output current of the battery flows in each of the battery cells 10-1, . . . , 10-n because the battery cells 10-1, . . . , 10-n are connected in series, with the charge transport taking place by electrochemical processes within the battery cells 10-1, . . . , 10-n, the failure of a single battery cell means, in the extreme, that the entire arrangement can no longer provide any current and therefore no electrical energy. In order to allow a threatened failure of a battery cell 10-1, . . . , 10-n to be identified in good time, a so-called battery management system 12 is normally used, which is or can be connected to both poles of each of the battery cells 10-1, . . . , 10-n and determines operating parameters such as the voltage and temperature of each battery cell 10-1, . . . , 10-n and, therefrom, their state of charge (SoC) at regular or selectable intervals. This means a high level of complexity with little flexibility at the same time for the electrical operating data of the battery system.
Further disadvantages of connecting a multiplicity of battery cells in series are:
1. Conditions are imposed for the operating voltage to be provided, the maximum current and the stored energy for various operating states of the device to be operated using the battery, which conditions can be combined only by coupling a greater number of battery cells than would actually be necessary to comply with the individual requirements. This increases the price, as well as the weight and volume of the battery system, which have a particularly disturbing effect in an electric car.2. The installation of the battery, that is to say the interconnection of the individual cells, takes place at high voltages up to 1000 V, because the voltages of the individual battery cells are added by connecting them in series, as a result of which the battery, individual cells or modules cannot be replaced in local workshops or, in the case of stationary use, can be carried out only with a special tool by especially trained skilled workmen. This results in a high level of logistic effort for maintenance of battery systems in the event of a fault.3. In order to switch the battery system to be free of voltage, that is to say to disconnect the actual battery from the load, circuit breakers 11-1 and 11-2 must be provided, which are typically in the form of contactors, and are very expensive for the high currents and voltages to be expected.